Quick and dirty lines to convert FLV files into MP4.

To do so, you need the package ffmpeg.

# Temporary move all the .flv files into a TMP directory
$ find . -type f -name "*.flv" -exec mv {} ../TMP/ \;

# Remove all spaces from the filename
$ cd ../TMP/
$ for file in *' '* ; do mv -- "$file" "${file// /_}" ; done

# Now, convert all the files from FLV to MP4 using ffmpeg
$ for file in `ls` ; do ffmpeg -i "$file" -c copy "${file//flv/mp4}" ; done

During the process, you might get some errors. You will see some .mp4 files with size 0 bites.

You can then try to remove the broken files and try converting re-encoding

# Find and delete files size 0
$ find . -type f -size 0 -print0 | xargs -0 rm

# Try re-encoding
$ for file in `ls *.flv` ; do ffmpeg -i "$file" -vcodec libx264 -acodec copy "${file//flv/mp4}" ; done